A disc drive includes a spindle motor, which, after spinning up to its specified speed, rotates one or more discs at a constant high speed. Information is written to and read from tracks on the discs through the use of an actuator assembly. The actuator assembly includes actuator arms, which extend towards the discs. Mounted on each of the actuator arms is a read/write head, which includes an air bearing slider enabling the head to fly in close proximity above the corresponding media surface of the associated disc.
Increases in storage media density have allowed disc drive manufactures to produce disc drives with large capacities and small form factors. For example, a five gigabyte disc drive having a smaller profile than a credit card, and a thickness less than a quarter-inch, is currently available. Such small form factor disc drives can be used in portable consumer electronics. In addition to increases in storage media density, reducing the size of other disc drive components allows further reduction in a disc drive form factor.
A typical spindle motor includes an outside stationary stator. The stationary stator includes a motor stator lamination having teeth wrapped in electric coils. The electric coils are supplied with current to produce a rotating magnetic field. The typical spindle motor also includes an inside rotor with poles to capture torque from the rotating field and output the torque to one or more discs in the drive via the disc drive spindle.
By reducing the height of a spindle motor, the overall design height in a disk drive can also be reduced. However, reducing the design height of a spindle motor, e.g., by reducing the winding volume on the stator teeth, generally results in a reduction of the electromagnetic performance of the spindle motor, e.g., it takes more current to produce the same torque. Because the electromagnetic performance of a spindle motor is an important quality factor in a disc drive, a desire to minimize the form factor of a disc drive must be balanced with a need for adequate electromagnetic performance of the disc drive's spindle motor.